Conventionally, pneumatic tires with an object of securing straight-line stability are known. For example, the pneumatic tire of Japanese Unexamined Patent Application Publication No. 2002-29216A comprises land portions defined by grooves that extend in a direction that intersects a tread width direction cross section in the tread portion. When viewed in a tread width direction cross section, the contact surface of the land portions is curved so as to protrude outward in the radial direction, and the apex of the contact surface closest to the profile line of the tread surface across the entire tread width is offset from the lateral center of the land portion toward one side edge of the land portion by from 0.1 to 0.4 times the width of the land portion.
Also, Japanese Unexamined Patent Application Publication No. 2004-122904A, for example, describes a pneumatic tire comprising a center land portion extending along the tire equator, an outer land portion extending in the shoulder portion, an intermediate land portion located between the center land portion and the outer land portion; these circumferential grooves defining a tread surface. Such a pneumatic tire when in a regular state of being mounted on a regular rim, inflated to a regular internal pressure, and having no load applied, when viewed in the tire meridian cross section including the tire axis, the radius of curvature R1 of the outer surface of the center land portion is greater than the radius of curvature R2 of the outer surface of the intermediate land portion, and the center of each radius of curvature R1, R2 lies at the same position.
In recent years, in line with increases in the performance of vehicles, pneumatic tires having both steering stability at high speeds and durability at high speeds when a camber is applied have been demanded. To cater to such a demand to ensure steering stability, a pneumatic tire such as that described in Japanese Unexamined Patent Application Publication No. 2002-29216A has been effective. The pneumatic tire of Japanese Unexamined Patent Application Publication No. 2002-29216A has a configuration in which a rib (land portion), formed in the tread portion, has a profile that, when viewed in the tire meridian cross section, protrudes outward in the tire radial direction more than the profile of the tread surface so that the rib has better contact with the ground. However, when a camber is applied to a vehicle, durability at high speeds tends to decrease. This is because, in the case of a negative camber, ribs located inward of the tire equator plane when the tire is mounted on a vehicle have a longer contact patch length than ribs located outward of the tire equatorial plane. Consequently, achieving both steering stability at high speeds and durability at high speeds when a camber is applied has been difficult.
The pneumatic tire described in Japanese Unexamined Patent Application Publication No. 2004-122904A is configured for enhanced durability. However, the pneumatic tire is for light trucks. Also, the pneumatic tire has a configuration in which, due to differences in the radius of curvature, a center land portion protrudes outward in the tire radial direction a great deal more than the intermediate land portion. Consequently, if the pneumatic tire was applied to a vehicle with a camber, durability of a center land portion would decrease and steering stability would also decrease.